我用 task-based operations 生成了一个代理.

应该如何使用 async/await 正确调用此服务(之后处理 ServiceClient 和 OperationContext)？

我的第一次尝试是:

public async Task<HomeInfo> GetHomeInfoAsync(DateTime timestamp)
{
    using (var helper = new ServiceHelper<ServiceClient, ServiceContract>())
    {
        return await helper.Proxy.GetHomeInfoAsync(timestamp);
    }
}

作为 ServiceHelper 创建 ServiceClient 和 OperationContextScope 并随后处理它们的类:

try
{
    if (_operationContextScope != null)
    {
        _operationContextScope.Dispose();
    }

    if (_serviceClient != null)
    {
        if (_serviceClient.State != CommunicationState.Faulted)
        {
            _serviceClient.Close();
        }
        else
        {
            _serviceClient.Abort();
        }
    }
}
catch (CommunicationException)
{
    _serviceClient.Abort();
}
catch (TimeoutException)
{
    _serviceClient.Abort();
}
catch (Exception)
{
    _serviceClient.Abort();
    throw;
}
finally
{
    _operationContextScope = null;
    _serviceClient = null;
}

但是，当同时调用两个服务并出现以下错误时，这会悲惨地失败:“此 OperationContextScope 正在与创建时不同的线程上处理。”

MSDN说:

Do not use the asynchronous “await” pattern within a OperationContextScope block. When the continuation occurs, it may run on a different thread and OperationContextScope is thread specific. If you need to call “await” for an async call, use it outside of the OperationContextScope block.

这就是问题所在!但是，我们如何正确修复它呢？

This guy did just what MSDN says :

private async void DoStuffWithDoc(string docId)
{
   var doc = await GetDocumentAsync(docId);
   if (doc.YadaYada)
   {
        // more code here
   }
}

public Task<Document> GetDocumentAsync(string docId)
{
  var docClient = CreateDocumentServiceClient();
  using (new OperationContextScope(docClient.InnerChannel))
  {
    return docClient.GetDocumentAsync(docId);
  }
}

我对他的代码的问题是，他从不在 ServiceClient 上调用关闭(或中止)。

我还找到了a way使用自定义 SynchronizationContext 传播 OperationContextScope。但是，除了它是很多“有风险的”代码这一事实之外，他还指出:

It’s worth noting that it does have a few small issues regarding the disposal of operation-context scopes (since they only allow you to dispose them on the calling thread), but this doesn’t seem to be an issue since (at least according to the disassembly), they implement Dispose() but not Finalize().

那么，我们在这里运气不好吗？是否存在使用异步/等待调用 WCF 服务并同时处理 ServiceClient 和 OperationContextScope 的可靠模式？也许来自 Microsoft 的某个人(也许是大师 Stephen Toub :))可以提供帮助。

谢谢!

[更新]

在用户 Noseratio 的大量帮助下，我想出了一些可行的办法:不要使用 OperationContextScope。如果您将它用于任何 these原因，请尝试找到适合您的情况的解决方法。否则，如果您真的非常需要 OperationContextScope，您将不得不想出一个捕获它的 SynchronizationContext 的实现，这似乎是 very hard (如果可能的话——这不是默认行为一定是有原因的)。

所以，完整的工作代码是:

public async Task<HomeInfo> GetHomeInfoAsync(DateTime timestamp)
{
    using (var helper = new ServiceHelper<ServiceClient, ServiceContract>())
    {
        return await helper.Proxy.GetHomeInfoAsync(timestamp);
    }
}

ServiceHelper 是:

public class ServiceHelper<TServiceClient, TService> : IDisposable
    where TServiceClient : ClientBase<TService>, new()
    where TService : class
{
protected bool _isInitialized;
    protected TServiceClient _serviceClient;

    public TServiceClient Proxy
    {
        get
        {
            if (!_isInitialized)
            {
                Initialize();
                _isInitialized = true;
            }
            else if (_serviceClient == null)
            {
                throw new ObjectDisposedException("ServiceHelper");
            }

            return _serviceClient;
        }
    }

    protected virtual void Initialize()
    {
        _serviceClient = new TServiceClient();
    }

    // Implement IDisposable.
    // Do not make this method virtual.
    // A derived class should not be able to override this method.
    public void Dispose()
    {
        Dispose(true);

        // Take yourself off the Finalization queue 
        // to prevent finalization code for this object
        // from executing a second time.
        GC.SuppressFinalize(this);
    }

    // Dispose(bool disposing) executes in two distinct scenarios.
    // If disposing equals true, the method has been called directly
    // or indirectly by a user's code. Managed and unmanaged resources
    // can be disposed.
    // If disposing equals false, the method has been called by the 
    // runtime from inside the finalizer and you should not reference 
    // other objects. Only unmanaged resources can be disposed.
    protected virtual void Dispose(bool disposing)
    {
        // If disposing equals true, dispose all managed 
        // and unmanaged resources.
        if (disposing)
        {
            try
            {
                if (_serviceClient != null)
                {
                    if (_serviceClient.State != CommunicationState.Faulted)
                    {
                        _serviceClient.Close();
                    }
                    else
                    {
                        _serviceClient.Abort();
                    }
                }
            }
            catch (CommunicationException)
            {
                _serviceClient.Abort();
            }
            catch (TimeoutException)
            {
                _serviceClient.Abort();
            }
            catch (Exception)
            {
                _serviceClient.Abort();
                throw;
            }
            finally
            {
                _serviceClient = null;
            }
        }
    }
}

注意该类支持扩展；也许您需要继承并提供凭据。

唯一可能的“陷阱”是在 GetHomeInfoAsync 中，您不能只返回从代理获得的 Task(这看起来很自然，为什么要创建一个new Task 当你已经有了一个)。那么，在这种情况下，您需要等待代理Task，然后然后关闭(或中止)ServiceClient，否则您将在调用服务后立即关闭它(同时通过网络发送字节)!

好的，我们有办法让它发挥作用，但如 Noseratio 所述，最好能从权威来源获得答案。

最佳答案

我认为一个可行的解决方案可能是使用自定义等待程序 通过OperationContext.Current 流动新的操作上下文。 implementation of OperationContext本身似乎不需要线程关联。这是模式:

async Task TestAsync()
{
    using(var client = new WcfAPM.ServiceClient())
    using (var scope = new FlowingOperationContextScope(client.InnerChannel))
    {
        await client.SomeMethodAsync(1).ContinueOnScope(scope);
        await client.AnotherMethodAsync(2).ContinueOnScope(scope);
    }
}

下面是 FlowingOperationContextScope 和 ContinueOnScope 的实现(仅略微测试):

public sealed class FlowingOperationContextScope : IDisposable
{
    bool _inflight = false;
    bool _disposed;
    OperationContext _thisContext = null;
    OperationContext _originalContext = null;

    public FlowingOperationContextScope(IContextChannel channel):
        this(new OperationContext(channel))
    {
    }

    public FlowingOperationContextScope(OperationContext context)
    {
        _originalContext = OperationContext.Current;
        OperationContext.Current = _thisContext = context;
    }

    public void Dispose()
    {
        if (!_disposed)
        {
            if (_inflight || OperationContext.Current != _thisContext)
                throw new InvalidOperationException();
            _disposed = true;
            OperationContext.Current = _originalContext;
            _thisContext = null;
            _originalContext = null;
        }
    }

    internal void BeforeAwait()
    {
        if (_inflight)
            return;
        _inflight = true;
        // leave _thisContext as the current context
   }

    internal void AfterAwait()
    {
        if (!_inflight)
            throw new InvalidOperationException();
        _inflight = false;
        // ignore the current context, restore _thisContext
        OperationContext.Current = _thisContext;
    }
}

// ContinueOnScope extension
public static class TaskExt
{
    public static SimpleAwaiter<TResult> ContinueOnScope<TResult>(this Task<TResult> @this, FlowingOperationContextScope scope)
    {
        return new SimpleAwaiter<TResult>(@this, scope.BeforeAwait, scope.AfterAwait);
    }

    // awaiter
    public class SimpleAwaiter<TResult> :
        System.Runtime.CompilerServices.INotifyCompletion
    {
        readonly Task<TResult> _task;

        readonly Action _beforeAwait;
        readonly Action _afterAwait;

        public SimpleAwaiter(Task<TResult> task, Action beforeAwait, Action afterAwait)
        {
            _task = task;
            _beforeAwait = beforeAwait;
            _afterAwait = afterAwait;
        }

        public SimpleAwaiter<TResult> GetAwaiter()
        {
            return this;
        }

        public bool IsCompleted
        {
            get 
            {
                // don't do anything if the task completed synchronously
                // (we're on the same thread)
                if (_task.IsCompleted)
                    return true;
                _beforeAwait();
                return false;
            }

        }

        public TResult GetResult()
        {
            return _task.Result;
        }

        // INotifyCompletion
        public void OnCompleted(Action continuation)
        {
            _task.ContinueWith(task =>
            {
                _afterAwait();
                continuation();
            },
            CancellationToken.None,
            TaskContinuationOptions.ExecuteSynchronously,
            SynchronizationContext.Current != null ?
                TaskScheduler.FromCurrentSynchronizationContext() :
                TaskScheduler.Current);
        }
    }
}

关于c# - 使用 async/await 调用 WCF 服务的模式，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/18284998/